The present invention relates to printing blankets of the type used in offset lithography, and more particularly to the use of a printing blankets having a low-resilience printing surface.
An offset printing blanket is used to transfer ink and fountain solution (primarily water) to paper or other substrates from a printing plate on a printing press. It can also serve a similar function in transferring other coatings including, for example, varnishes. As used herein, the term xe2x80x9cinkxe2x80x9d refers to any printing fluids or coating. Also, the term xe2x80x9cprinting blanketxe2x80x9d as used herein refers to any of the forms that may achieve the same basic function of printing. Printing blankets are typically wrapped around a cylinder and the paper is either sheet feed or fed on a web between the rollers.
The role of a printing blanket is to transfer dots of ink and water films from a printing plate in an offset press to a substrate (typically but not exclusively paper). The surface of the blanket must have a natural affinity (i.e., adhesion) to ink in order to pick up ink from a printing plate. When that ink is then pressed into contact with the substrate, some of the ink comes off of the blanket and wets the surface of the substrate and printing is achieved. Not all of the ink on the blanket is transferred to the substrate due to the natural tendency for the ink closest to the blanket to adhere to the blanket surface.
In this connection, only 30% to 40% of the ink on a blanket will typically transfer to the substrate in printing. That means that some ink remains on the blanket and is over-coated with additional ink on subsequent rotations of the printing press. The ink that stays on the blanket will increase in tack level (i.e., adhesiveness) the longer it stays on the blanket, further limiting the amount of ink that may be transferred to the substrate. This phenomenon typically limits print quality since ink that stays on the blanket is also continuously exposed to water. The water emulsifies the ink in time and may degrade the color brightness of the ink, and also allow it to be printed in areas that were not intended to contain printed images (the non-image area). Also, on each revolution of the printing press the ink left behind on the blanket is exposed to compression and shear forces when pressed against the substrate or against the printing plate. These contact areas are known as xe2x80x9cnips.xe2x80x9dThe longer the ink stays on the surface, therefore, the more opportunity there is to gradually spread or distort the ink, widening the printed dot, and creating a slurring of the print and dot gain.
Offset printing blankets typically have a multi-layer construction that comprises layers of fabric, foam and reinforcing rubber layer(s) (collectively known as the blanket carcass and stabilizing layer) and the topmost layer called the surface layer. Good quality printing is generally dependent on the overall construction or design of the printing blanket as well as the materials and topographical characteristics of the layer used for the printing surface.
The surface layer plays a critical role in transferring the print impression from the lithographic printing plate to the printed substrate and is consequently required to have a good balance of surface wettability and affinity for the oil-based ink and water-based fountain solution used in the lithographic printing. The surface layer must be able to withstand repeated contact with the ink and fountain solutions and must also have good compressibility and resiliency. That is, the blanket must be able to be compressed between the two cylinders, but have sufficient resiliency to return to its original thickness quickly enough to be ready for the next impression. An important property of the blanket is that by its nature and structure it must permit the development of even, uniform printing pressures in order to achieve a quality finished product. Furthermore, the blanket must have a firm, non-extendible base in order that it may be held under tension on the offset cylinder without stretching or becoming distorted in any way.
There are many types of printing blanket designs available including sheets cut from rolls that are then mounted onto a printing press via adhesives or by various clamping mechanisms including but not limited to the use of blanket bars, endless or gapless tubular constructions known generally as xe2x80x9cblanket sleeves,xe2x80x9d metal backed blankets, etc. All of these designs and uses of blankets must be capable of transferring ink generally printed as small dots with a minimum of blurring of the edges of the dot in order to produce a xe2x80x9csharpxe2x80x9d image. Any surface texture to the printing blanket tends to allow the ink to spread on the surface to some degree and reduce the sharp visual appearance of the printed dots or the otherwise straight edges to printed or coated areas. The ideal limit to achieving the sharpest printing would be to use a completely smooth and non-textured surface. However, the inks are required to have a level of tackiness that provides some bonding to the plate, blanket, and substrate to be printed on. This tackiness makes release from an extremely smooth surface more difficult. Release of the ink from the blanket surface is enhanced by increasing the texture (i.e., roughness) of the surface. Increasing surface roughness, however, limits the printing sharpness that can be achieved. Additionally, it is well known in the art that a very smooth surface will create a mottled appearance to xe2x80x9cprinted solids,xe2x80x9d that is printed areas that are meant to have a complete covering of ink. Smooth surfaces generally release ink poorly and unevenly on a microscopic scale. Thus, a textured surface will typically produce a more visually appealing smoother looking printed solid by improving on ink release.
The most common technique in the art that attempts to maximize print quality overall by balancing the desire to produce sharp printed dots and visually appealing solids is by the use of a buffed surface. As used herein, the term xe2x80x9cbuffedxe2x80x9d refers to the surface of a blanket that is finely ground to achieve a micro-textured surface topography that releases ink well but is meant to limit the distortion of the sharp dot edges. Alternative approaches include molding the surface against a casting medium (commonly a release paper) or manufacturing the surface with small holes or ink wells. Both of these techniques are limiting in the quality of the solids printed or in the sharpness of the dot printed. The average roughness (Ra value) of these buffed surfaces is, however, typically 0.5 microns or greater.
It is generally accepted in the art that the surface layer of a printing blanket must be made of a highly resilient material that is ink (and usually fountain solution) receptive. Typically these surfaces are made from a variety of rubber or rubber-like polymeric materials that are formulated along with other features for their ink and water receptivity and resilience. High resilience is viewed as a requirement due to the very small amount of time (micro-seconds) available in a printing nip between blanket and plate or blanket and substrate being printed. It is believed that high resilience provides the physical responsiveness required to allow the blanket surface to quickly conform to the roughness of the printed substrate in particular and to transfer good quality ink dots and solids. Additionally, it is believed that, a low resilience printing layer would require a highly textured surface which then would adversely effect print sharpness. The reasoning is that a highly textured surface would carry a thicker ink film and so compensate for the loss of contact time in the printing nip due to the poor ability of the low resilience printing layer to stay in intimate contact with the printing substrate.
High resiliency of the blanket""s printing layer, however, has become a problem with increased demand for higher speed printing. For example, press speeds in offset printing have risen from approximately 1200 ft/min to 3000 ft/min in the last 15 years. The desire for increases in speed has come from the need for printers to have increased productivity to remain competitive. As printing press speeds have increased, it has become more difficult to achieve high print quality. Higher printing speeds mean that the press cylinders are spinning faster, thus, the ink used must have a higher adhesion or xe2x80x9ctackxe2x80x9d level to stick to the blanket, plate, and inking cylinders, otherwise the ink is sprayed off of the cylinders and can not print well. The problem is that, as described above, as ink tack levels are increased, it becomes increasingly harder to then transfer the ink from the blanket onto the paper or other printing substrate. Thus, the need exists then for improved release at higher printing speeds.
A highly resilient printing surface on a blanket will more easily elongate outward as the ink is pulled off of the blanket and, therefore, delay putting tension on the ink that is being transferred to the printing substrate. The result is to hinder release of the ink. A low resilient surface, however, will not move quickly in being pulled upward from the blanket surface and, so, will release ink faster. As press speeds are increased a highly resilient surface will continue to respond quickly to the forces the ink is putting on the surface and will make ink release more difficult as ink tack levels are increased for the higher press speeds. It is speculated that a low resilient surface will be unable to keep up with the faster motions the ink is experiencing at higher speeds and so will put added force on the ink, forcing it to release easily. Thus, it is believed that, ultimately, a highly resilient blanket surface will not act to improve ink release at higher speeds and a low resilient surface will. This conclusion, however, is counter to the accepted art that has driven towards increased resilience in blanket surfaces and other components for higher speed applications. The traditional belief is that higher resilience is needed for the blanket to respond to the faster dynamics of high speed printing. In actuality, the result of the use of highly resilient blanket materials leads to a very limited operating window on press as to acceptable ink tack levels and has lead to a reduction in print quality on high speed printing.
On slower speed, sheet fed presses, for example, the benefit of low resilient surfaces are also seen since the printing nip is still dynamic and an improvement in release is still observable. In particular, when the printed sheet is peeled off of the printing blanket, improved release of the ink allows for a reduction in the contact time between blanket, ink, and paper and so, reduces distortions in the printed dots. That improvement in release has allowed heretofore impractical degrees of surface smoothness to be used that would otherwise have provided very poor ink release, giving poor print quality. A highly smooth surface provides very sharp, smooth printed dots that visibly increase the image resolution that is possible for offset printing.
High resiliency of the blanket""s printing layer is also an issue with respect to the substrate to which the ink is being transferred. For substrates having a rough surface such as, for example, rough paper stocks, good ink transfer and release is fairly easy to achieve since the rough surface helps to break the ink dots that are being printed and, so, release the ink. Glossy paper, however, is very smooth and makes the release more difficult. The desire in the art is to use a very smooth blanket surface to print on a very smooth paper to achieve the highest print quality. Release of the ink then becomes very difficult with traditional, resilient blanket surfaces and the ultimate print quality is limited. Having a low resilient surface allows the resolution possible in offset printing to no longer be limited by the roughness of the surface of the blanket, only by the choice of paper used.
Thus, there is a need in the art for a printing blanket having a surface that has the ability to print high quality images on a variety substrates having different textures and at high speeds.
The inventor has found that, independent of the materials used to form the printing surface of the present invention, a high quality image that can be printed on a variety of substrates having different textures at high speeds can be achieved when the printing surface of the printing blanket exhibits certain physical characteristics. Surprisingly, it has been found that a smooth, low-resilience surface described herein has been shown to transfer a much higher percentage of ink applied to it. By transferring more ink, the ink has less opportunity to be emulsified with water, resulting in higher print quality. With less ink remaining on the surface of the blanket there is also less distortion of the dots that is possible due to the forces in the printing nips and sharper, higher resolution images can be produced.
Specifically, an offset printing blanket is provided comprising a printing layer and a base, wherein the printing layer has a Shore Resilience of less than about 40% and an average surface roughness of less than about 0.5 microns.
A method for achieving superior ink-transfer onto a substrate according to the present invention is provided comprising the steps of providing an offset printing blanket comprising a printing layer and a base, wherein the printing layer has a Shore Resilience of less than about 40% and an average surface roughness of less than about 0.5 microns. Ink is transferred to said offset printing blanket from an image-bearing printing element. The ink is then transferred from said offset printing blanket onto a substrate, wherein at least about 75% of said ink is transferred from said offset printing blanket to said substrate.
According to another aspect of the present invention, a method is provided for determining the suitability of an off-set printing blanket to transfer ink based upon resilience measurements of the blanket""s printing surface. The method comprises providing a printing blanket having an elastomeric printing surface, determining whether the resilience of the printing surface is below about 40%, and utilizing the printing blanket if the resilience of the printing surface is below about 40%.